1. Field of the Invention
This invention relates to stabilized lubricant compositions comprising lubricating oil and a class of synergistic mixtures of organophosphites and conventional antioxidants that impart excellent anti-oxidative stabilities and are particularly suitable for use in a high temperature iron-catalyzed environment.
2. Description of Related Art
Lubricating oils, when exposed to heat and oxygen (air), which are ubiquitously present during their manufacture, transportation, storage, or use, will oxidize to form undesirable oxidation by-products that contribute to an increase in total acidity, formation of gums, discoloration, polymerization, rancidity, and/or odor. As a consequence, loss of designated physical and tribological properties of the oils may occur. Conventional antioxidants, including aromatic secondary aminic antioxidants and phenolic antioxidants, are effective, at least to some extent, in controlling the oxidation of lubricating oils and are being widely used. The performances of the conventional antioxidants are generally satisfactory when the lubricants to be protected are used under relatively mild conditions without prolonged exposure to elevated temperatures and contaminants. In more extreme service environments, especially those contaminated with catalytic transition metals, such as iron, the decomposition rates of lubricants may increase so dramatically that even at higher concentrations, the conventional antioxidants do not retard oxidation effectively. This aspect is discussed in more detail by Miller, H., Plastics Additives Handbook, 4th Edition, Metal Deactivators, 1993, pages 106-128.
An effective method to address the aforementioned issue of iron-catalyzed oil oxidation is the use of metal deactivators that can counteract the catalytic effects from iron and other transition metals. However, as stated in the EP Publication No. 0 316 610 A1, the addition of metal deactivators to lubricants has given rise to a problem of decreasing the critical anti-seizure and antiwear properties of the antiwear/extreme pressure agents that are commonly used in lubricant formulations.
Lubricant compositions containing various aromatic secondary amines and substituted phenols are widely known in the art. The use of organophosphites as stabilizers for various lubricating substances is also known, although to a lesser extent.
U.S. Pat. No. 3,556,999 discloses hydraulic fluid compositions, particularly automatic transmission fluid compositions, containing a major amount of lubricating oil and a minor amount of each of (A) a phosphite or disubstituted phosphate, (B) a substituted phenol or an aromatic secondary amine and (C) an oil-soluble dispersant copolymer containing N-vinyl-2-pyrrolidone.
U.S. Pat. No. 3,652,411 discloses a polyglycol base lubricant containing, in minor proportion, as a stabilizer, a mixture comprising: a substituted amine, an aliphatically substituted phenol, and organic phosphate, a polyhydroxyquinone, a benzotriazole, an amine salt and a substituted organic phosphite.
U.S. Pat. No. 3,923,672 discloses a lubricating oil composition said to be particularly suitable for use in steam turbines or gas turbines. The turbine oil composition comprises a major amount of a mineral or synthetic hydrocarbon base oil and an effective amount of a combination of the following materials: triphenyl phosphite or a trialkyl-substituted phenyl phosphite; diphenylamine or alkylated diphenylamine; phenyl α-naphthylamine, phenyl β-naphthylamine, alkyl or alkaryl substituted phenyl α-naphthylamine, or alkyl or alkaryl substituted phenyl β-naphthylamine; benzotriazole or alkyl-substituted benzotriazole; partial ester of alkyl or alkenyl succinic anhydride. In a preferred aspect, the turbine oil composition contains additionally an effective amount of a copolymer of N-vinyl-2-pyrrolidone and an α-olefin.
U.S. Pat. No. 5,124,057 discloses lubricant compositions in which a synergistic combination of low-volatility tri-substituted phosphite and selected substituted isocyanurate phenolic stabilizers provide antioxidant qualities to lubricating oils selected from hydrotreated oils, poly-α-olefin oils, paraffinic white oils and mixtures thereof.
U.S. Pat. No. 5,232,614 discloses that substituted para-phenylene diamines have been found to be effective antioxidants capable of protecting crankcase lubricating oils from thickening and sludge formation after prolonged exposure to oxygen at elevated temperature.
U.S. Pat. No. 6,172,014 discloses an improved method of reducing compressor gas leakage by providing a compression cylinder with a lubricant comprising less than about 1% of a synergistic mixture of antioxidants.
U.S. Patent Publication No. 2003/0171227 discloses stabilising compositions for lubricant base stocks and lubricant formulations that are composed of a mixture of (a) at least one aromatic aminic amine antioxidant optionally blended with at least one hindered phenolic antioxidant and (b) at least one neutral organo phosphate or phosphite, optionally blended with at least one acid organo phosphate or phosphite. It is said that these stabilising composition mixtures are characterised by their stabilising capacity which is considerably higher than that of either the single antioxidants or the single phosphate or phosphite additives and can be used in all fields where the single components of the mixtures are generally used and where deterioration due to oxidation processes takes place.
The disclosures of the foregoing are incorporated herein by reference in their entirety.